Hydrogen sulfide is a major source of pollution of gas streams since it is liberated as a waste by-product in a number of chemical processes, such as sulfate or kraft paper pulp manufacture, viscose manufacture, sewage treatment, the production of organic sulfur compounds, as well as during petroleum refining and in the production of natural gas and combustible gases from coal, such as in coking operations. Hydrogen sulfide is also present in geothermal steam, which is captured for use in power generating plants.
To eliminate these polluting sulfur gases the art has developed several oxidation-reduction (“redox”) processes that use an aqueous chelated metal catalyst solution for removing hydrogen sulfide from a gas stream. In those prior art processes a hydrogen sulfide-containing gas, known as “sour gas,” is contacted with a chelated metal catalyst to effect absorption. Subsequent oxidation of the hydrogen sulfide to elemental sulfur and concurrent reduction of the metal to a lower oxidation state also occurs. The catalyst solution is then regenerated for reuse by contacting it with an oxygen-containing gas to oxidize the metal back to a higher oxidation state. The elemental sulfur is continuously removed from the process as a solid product with high purity. Illustrative, but not exclusive, of these oxidation-reduction processes is the description contained in U.S. Pat. No. 4,622,212 (McManus et al.) and the references cited therein.
In order to return the “spent” liquid redox catalyst solution to its original oxidation level so it can be recycled for subsequent use in the process, oxygen must be supplied to the spent redox catalyst solution. This is typically accomplished using various mechanical apparatus, including well known tank spargers that use compressed air as the source of oxygen. A problem with these prior art air spargers is that they require large volumes of excess air due to the inherently low mass transfer rates of the oxygen into the redox catalyst solution. Higher excess air results in higher equipment and utility costs, thus reducing the overall process economics.
Up until now, the art has failed to come up with an apparatus and/or method of increasing the mass transfer rates of oxygen into the redox catalyst solution. Our invention solves this problem by utilizing membrane technology where an oxygen containing gas, typically air, diffuses through the membrane walls and forms extremely small bubbles of gas in the redox catalyst solution, thus significantly increasing the mass transfer rates and minimizing the amount of excess air needed to regenerate the redox spent catalyst solution. Such an apparatus and associated process represents an extremely economical method of regenerating the catalyst, and consequently, minimizing capital and operating costs. These and other advantages will become evident from the following more detailed description of the invention.